Semiconductor device for directly converting radioisotope emissions into electrical power

ABSTRACT

A device for producing electricity. In one embodiment, the device comprises a doped germanium or a doped GaAs substrate and a plurality of stacked material layers (some of which are doped) above the substrate. These stacked material layers, which capture the beta particles and generate electrical current, may include, in various embodiments, GaAs, InAlP, InGaP, InAlGaP, AlGaAs, and other semiconductor materials. A beta particle source generates beta particles that impinge the stack, create electron-hole pairs, and thereby generate electrical current. In another embodiment the device comprises a plurality of epi-liftoff layers and a backing support material.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application and claimspriority to the continuation-in-part application Ser. No. 15/790,713,filed on Oct. 23, 2017 (Attorney Docket 11432-008CIP), now U.S. Pat. No.______, which claims priority to application Ser. No. 14/623,861, filedon Feb. 17, 2015 (Attorney Docket Number 11432-008), now U.S. Pat. No.9,799,419, which claims priority under 35 U.S.C. 119(e) to theprovisional patent application filed on Feb. 17, 2014 and assignedapplication No. 61/940,571. Each of these non-provisional applicationsand the provisional patent application are incorporated in theirentirety herein.

BACKGROUND OF THE INVENTION

The direct conversion of radioisotope beta (electron) emissions intousable electrical power via beta emissions directly impinging on asemiconductor junction device was first proposed in the 1950's. Incidentbeta particles absorbed in a semiconductor create electron-hole-pairs(EHPs) which are accelerated by the built-in field to device terminals,and result in a current supplied to a load. These devices are known asdirect conversion semiconductor devices, beta cells, betavoltaicdevices, betavoltaic batteries, isotope batteries, betavoltaic powersources, betavoltaic(s) etc. These direct conversion devices promise todeliver consistent long-term battery power for years and even decades.For this reason, many attempts have been made to commercialize such adevice. However, in the hopes of achieving reasonable power levels, theradioisotope of choice often emitted unsafe amounts of high energyradiation that would either quickly degrade semiconductor deviceproperties within the betavoltaic battery or the surrounding electronicdevices powered by the battery. The radiated energy may also be harmfulto operators in the vicinity of the battery.

As a result of these disadvantages, and in an effort to gain approvalfrom nuclear regulatory agencies for these types of batteries, thechoice for radioisotopes has been limited to low-energy beta (electron)emitting radioisotopes, such as nickel-63, promethium-147, or hydrogen-3(tritium). Since promethium-147 is regulated more stringently andrequires considerable shielding, whereas nickel-63 has a relatively lowbeta flux, tritium has emerged as a leading candidate for such a batterydevice.

Tritium betavoltaic batteries, sometimes referred to as tritiumbetavoltaic devices or tritium direct conversion devices, have beenpromoted during the last thirty years. Tritium is listed in variousregulatory guideline documents as being in the low toxicity group ofradioisotopes producing only low beta energy emissions that can beeasily shielded with as little as a thin sheet of paper. Tritium has along track record of commercial use in illumination devices such as EXITsigns in commercial aircraft, stores, school buildings and theatres. Itis also widely used in gun sights and watch dials, making it an idealpower source for the direct conversion devices. Unfortunately, tritium'sbeta emissions are so low in energy that it is has been difficult toefficiently convert these emissions into usable electrical power foreven the lowest power applications, such as powering an SRAM memory toprevent loss of stored data.

Several attempts have been made to produce useful current from a tritiumbetavoltaic battery. For example, polycrystalline or amorphoussemiconductor devices have been considered for tritium betavoltaicbatteries based on the assumption that such devices would allowbatteries to be fabricated at a reduced cost. It is assumed that thesedevices could be manufactured in a thin-film like fashion and thattritium could be embedded within the polycrystalline or amorphousdevices. However, this approach is extremely inefficient with respect tothe beta energy emissions entering the semiconductor (less than about1%). The main reason for this low semiconductor conversion efficiency isthe high dark current that acts as a negative current relative to thecurrent generated by the beta emissions. This high dark current competeswith the betavoltaic current produced by collection of EHPs created viathe tritium beta particles impinging on the semiconductor. In short, thepolycrystalline and amorphous semiconductors have a high number ofdefects resulting in recombination centers for the EHPs, which in turnsignificantly reduce the betavoltaic current and lead to very lowefficiency for the battery.

The best results for tritium betavoltaics have been achieved with singlecrystal semiconductor devices. Recent attempts have involved singlecrystalline semiconductor devices with a tritium source such as atritiated polymer, aerogel or tritiated metal hydride placed in directcontact with a semiconductor junction device. Single crystallinesemiconductors have longer carrier lifetimes and fewer defects resultingin much lower dark currents. Representative efficiencies for tritiumbetavoltaic batteries were published in a reference text entitled:“Polymers, Phosphors and Voltaics for Radioisotope Microbatteries”edited by K. Bower et al. Single crystal semiconductor devices wereexposed to tritium metal hydride sources on top of the semiconductors.Several homojunction (e.g., a conventional junction occurring at theinterface between an n-type (donor doped) and p-type (acceptor doped)semiconductor, such as silicon, also referred to as a p-n junction)semiconductor cells were utilized with the following results:

Silicon Cells:

-   -   Short Circuit Current=18.1 nA/cm̂2    -   Open Circuit Voltage=0.162    -   Fill Factor=0.513    -   Tritiated Titanium Source=0.23 microwatts/cm̂2    -   Efficiency=1.3%

Aluminum Gallium Arsenide (AlGaAs) Cells:

-   -   Short Circuit Current=58 nA/cm̂2    -   Open Circuit Voltage=0.62    -   Fill Factor=0.751,    -   Power=27 nW/cm·2    -   Tritiated Titanium Source=0.48 microwatts/cm̂2,    -   Efficiency=5.6%

Silicon cells are a preferred choice due to their low cost. However,their low efficiency makes them a poor choice for even the lowest powerapplications, such as for use in SRAM memory devices. The performance ofthe AlGaAs homojunction cell is attractive with one of the higherreported efficiencies and would be suitable for powering an SRAM memorydevice, in particular by stacking of tritiated metal hydride layers andAlGaAs homojunction cells. However, AlGaAs homojunctions cells aredifficult to reproduce consistently with uniform dark currents across asemiconductor device due to the oxidation of the aluminum. As a result,AlGaAs is also an expensive option to scale up.

Safety concerns over containment of the tritium based betavoltaicbattery have emerged as another obstacle to commercialization of atritium battery. In commercially available products such as tritiumillumination devices (e.g. EXIT signs, gun sights and watch dials), thetritium is in gaseous form and contained within a glass vial. Manyaccidents involving tritium release due to the breakage of the tritiumvials in EXIT signs have caused public concerns and resulted in costlyclean-up operations. The use of hermetically sealed packages has reducedthese concerns somewhat.

A tritium betavoltaic battery utilizing solid-state tritium metalhydride sources presents a lower exposure risk than gaseous tritiumdevices. However, the solid-state tritium metal hydride still involves aminiscule amount of tritium release when open to the environment at roomtemperature. Although several tritium based batteries have been proposedincluding direct conversion devices built within an integrated circuit,a method of effectively hermetically packaging the battery containingthe tritium metal hydride continues to be problematic.

A major obstacle to hermetically sealing this type of battery is therisk associated with using a sealing process that involves hightemperatures, i.e., above 200-300° C. At these temperatures, tritium isreleased from the metal hydride, possibly leading to battery failureafter sealing, or worse, causing tritium exposure at the sealingmanufacturing facility and to the operator of the sealing equipment.

The texturing of a direct conversion semiconductor device to increasethe surface area exposed to radiation emission has been proposed severaltimes in the past. For example, on page 282 of the book entitled“Polymers, Phosphors and Voltaics for Radioisotope Microbatteries”edited by K. Bower et al., the use of porous silicon and tritiuminserted into porous silicon holes was proposed as a means of increasingthe surface area of the semiconductor device by 20 to 50 times, incontrast to the original planar semiconductor surface area.

The following published patent applications and patents each propose amethod of increasing the surface area of the semiconductor by texturedgrowth of the semiconductor or a post-growth texturing method:

US Patent Application Publication 2004/0154656

US Patent Application Publication 2007/0080605

U.S. Pat. No. 7,250,323

U.S. Pat. No. 6,949,865

Central to this approach is the hope that an increase in surface areaexposed to radioisotope emissions will increase the power per unitvolume of the direct conversion semiconductor device. The goal of thisapproach is to not only reduce the size of the direct conversion device,but also to potentially reduce the cost associated with producing theequivalent surface area in a planar semiconductor device.

The problem with such an approach arises when a relatively low energyradioisotope such as tritium is used. In this case, the incident poweris quite small per unit of exposed area and the dark current of thesemiconductor device is a very significant factor in the overallefficiency. Unfortunately, alterations to the semiconductor surface riskincreasing lattice defects, resulting in a high number of recombinationcenters for EHPs. This creates a direct conversion semiconductor devicewith a low open circuit voltage and reduced short circuit current, witha resulting low overall efficiency.

Generally, it is preferable to use a single crystal semiconductormaterial where device defects are minimized and the dark current issufficiently low so that power can be efficiently produced.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more easily understood and the advantagesand uses thereof more readily apparent when the detailed description ofthe present invention is read in conjunction with the figures wherein:

FIG. 1 is a representation of an InGaP homojunction in contact with atritiated metal hydride source connected to a load.

FIG. 2 illustrates a physical structure of an InGaP homojunction devicefor tritium betavoltaic conversion, illustrating the individualsemiconductor layers.

FIG. 3 is an electron band diagram for an InGaP device.

FIGS. 4 and 5 each depict a general configuration of differentembodiments of betavoltaic device layers; in certain embodiments alldepicted layers many not be present.

FIG. 6 illustrates one embodiment for stacking a plurality of n/p (orp/n) cells in series using uni-directional beta sources.

FIGS. 7A and 7B illustrate a series connection of a p/n and an n/p cell.

FIGS. 8A and 8B illustrate an embodiment for stacking a plurality of n/p(or p/n) cells in parallel with bi-directional beta sources.

FIG. 9 illustrates a parallel connection of units that are stackedvertically comprised of unidirectional (or bi-directional) beta sources.

FIGS. 10A and 10B illustrate a seal lid for use with the device of thepresent invention.

FIGS. 11A-11C illustrate several views of a package for sealing abetavoltaic battery of the present invention.

FIG. 12 illustrates a betavoltaic device with a contact/conductorpresented on the front surface and electrically connected to a metalbacking/contact on a rear or bottom surface of the device.

FIGS. 13A-13D illustrate steps associated with an epitaxial lift-off(ELO) process.

FIGS. 14A-14D illustrate steps associated with a substrate removalprocess.

FIG. 15 illustrates another embodiment comprising two beta sources andtwo semiconductor material layers.

In accordance with common practice, the various described features arenot drawn to scale, but are drawn to emphasize specific featuresrelevant to the invention. Like reference characters denote likeelements throughout the figures and text.

DETAILED DESCRIPTION OF THE INVENTION

Before describing in detail the particular methods and apparatusesrelated to tritium direct conversion semiconductor devices, it should beobserved that the present invention resides primarily in a novel andnon-obvious combination of elements and process steps. So as not toobscure the disclosure with details that will be readily apparent tothose skilled in the art, certain conventional elements and steps havebeen presented with lesser detail, while the drawings and thespecification describe in greater detail other elements and stepspertinent to understanding the invention.

The following embodiments are not intended to define limits as to thestructure or method of the invention, but only to provide exemplaryconstructions. The embodiments are permissive rather than mandatory andillustrative rather than exhaustive.

The present invention is described in the context of a tritium directconversion semiconductor device comprising a single crystalsemiconductor. In preferred embodiments, the device exhibits arelatively low dark current and relatively high efficiency for theconversion of tritium's beta emissions into electrical power. It shouldbe understood that the high efficiency and longevity (e.g. over 10years) of the various device structure embodiments can also be attainedusing other candidate radioisotopes (e.g., promethium-147 andnickel-63), or combinations of radioisotopes, wherein the end-product isan electron or beta particle that impinges on a semiconductor material.Such devices can be formed on various substrate materials, such asgallium arsenide and germanium.

One embodiment of the present invention proposes a novel use of anIndium Gallium Phosphide (InGaP) homojunction semiconductor 8 (alsoreferred to as a betavoltaic junction, and comprising a plurality ofdoped semiconductor layers) in conjunction with a tritiated metalhydride source 10, as illustrated in FIG. 1, for supplying power to aload 12. The tritiated metal hydride source (e.g., scandium tritide,titanium tritide, palladium tritide, magnesium tritide, lithium tritide,or any combination thereof etc.) is in direct contact with (or proximateto) the semiconductor to generate electrical power at an efficiency of7.5% or higher with respect to the beta electrons impinging on theIndium Gallium Phosphide homojunction. InGaP is one of the larger bandgap materials and therefore can be advantageously used in a betavoltaicbattery.

One embodiment of the concept illustrated in FIG. 1 uses a compositionof the Indium Gallium Phosphide homojunction comprisingIn_(0.49)Ga_(0.51)P (subsequently referred to as InGaP). The band gap ofthis semiconductor is 1.9 eV and the materials production technology hasbeen well developed by the solar cell industry. The technology alsolends itself to high quality growth with a low density of latticedefects and low dark current characteristics. In addition, InGaP may bemass produced with a high yield due to the maturity of its manufacturingprocesses, thus lowering the cost of tritium betavoltaic batteries basedon the InGaP homojunction. InGaP, (and other III-V device structures)may be grown by metal-organic-chemical-vapor-deposition (MOCVD),molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), or throughother techniques known by those skilled in the art.

The embodiments described herein present novel and non-obvious featuresthat allow efficient conversion of tritium beta flux into electricalpower. FIGS. 2 and 3 illustrate the physical structure (doped layers)and electron band diagram, respectively. Each layer is lattice-matchedto a doped gallium arsenide (GaAs) substrate (germanium can also be usedas the substrate material, as described elsewhere herein) so that thenumber of dislocations generated by growth of the individual layers isminimized.

In general, beta particles radiate outward randomly in all directionsfrom the source. The beta particles in FIG. 2 are released by thetritiated metal hydride source (shown in FIG. 1 but not in FIG. 2) andare represented by dots, whereas the arrowheads represent one directionof motion of the beta particles emanating from the source.

FIG. 2 illustrates the individual layers of the nip homojunctionsemiconductor 8, comprising, from the bottom:

-   -   a pGaAs substrate (notation for a p-type GaAs substrate)    -   a pGaAs layer (one or more layers grown to establish a matching        crystal structure)    -   a p+InGaP layer (a back-surface field reflector or minority        carrier reflector; this layer reflects electrons, i.e., minority        carriers in a p-doped material)    -   a pInGaP layer (base)    -   an intrinsic InGaP layer (for preventing diffusion of dopants        between the p-doped and n-doped layers)    -   an nInGaP layer (emitter)    -   an nInAlP layer (window layer having a lattice structure that is        closely matched to the nInGaP below and the cap layer above that        allows electrons to pass to the cap layer; this window layer        reflects holes (i.e., minority carriers in an n-doped material)        back to the emitter)    -   an nGaAs cap layer (highly doped in one embodiment)

There are several features of this structure that allow efficientbetavoltaic energy conversion:

(a) A high quality, large band gap semiconductor junction resulting in ahighly efficient device; generally, any material having a band gapgreater than about 1.8 eV is considered a large band gap in the contextof this invention. InGaP is considered a wide bandgap material with aband gap of about 1.89 eV; the bandgap of InAIP is about 2.35 eV.

(b) A back-surface field reflector layer (in one embodiment ahighly-doped p⁺InGaP layer) that reflects electrons back onto thejunction field. The back-surface field reflector layer can also beformed by p-type: InAlP, AlAs, AlAsP, InAlGaP, ZnSe, a pseudomorphiclayer, or other materials known in the art.

(c) A lattice-matched n-type InAlP window layer that reflects holes (dueto the presence of a small electric field) back to the emitter layer (tothe junction), which also contributes to a desired low dark current.This window layer can also be formed with a highly-doped n+ InAlGaP,ZnSe, AlAs, n+InGaP, AlAsP, a pseudomorphic layer, or other materialsknown in the art;

(d) A GaAs cap layer having a thickness of a few hundred Angstroms orless, and covering the top surface;

(e) a 1000 to 3000 Angstrom layer of intrinsic InGaP to act as a bufferto diffusion of the p-type base dopant (usually Zn) into the n-typeemitter region.

The use of features (a), (b) and (c) in a tritium betavoltaicapplication is considered novel. The novel features (d) and (e) may beimportant for betavoltaic conversion, but they are not necessarily usedfor photovoltaic energy conversion. All of these features create a lowdark current that is required for efficient betavoltaic energyconversion. The novel lattice-matched InAIP window layer (with a largerband gap) prevents the formation of dislocations at the InAlP(window)-InGaP (emitter) interface, which would increase the darkcurrent. The GaAs cap layer prevents oxidation of the InAlP windowlayer, as this oxidation can introduce defects that provide sites forEHP recombination at the InAlP—InGaP region. This cap layer, thereforeaugments hole reflections at that interface. The GaAs cap layer does notabsorb a significant percentage of the beta flux, and therefore its usecan be tolerated. The cap layer material may comprise other group III-Vmaterials or combinations of III-V materials that have similarfunctionality.

In one embodiment, the cap layer is about 50-500 Angstroms thick, orless. In one embodiment, the gridlines are first deposited on top of athick (for example, about 3000 to 10000 Angstroms) cap layer and thenthe cap layer is removed by an etch process, that is, except for caplayer material under the grid lines. To retain a thinner portion of thiscap layer material, the etch process can be timed so that about 50-500Angstroms of cap layer material remains.

Alternatively, an etch stop layer can be formed and located such thatwhen the etchant reaches the etch stop layer, about 50-500 Angstroms ofcap layer material remains. For example, according to one method, a thin50-500 Angstrom cap layer, for protecting the window, uses a selectiveetch of the GaAs cap layer that stops at a thin InGaP layer leaving athin layer of InGaP over the 50-500 Angstrom GaAs cap layer, and ifdesired, the InGaP layer may then be selectively etched down to the GaAscap layer.

In another embodiment, a thick cap layer (for example 500-10000Angstroms or greater) may be grown, and a conductive contact material(e.g., metalcontacts/gridlines/ring/terminals/pads/points/epoxy(s)/solders, etc.)may be placed or deposited on the thick cap layer. The cap layer may bechemically etched, physically ablated, or otherwise removed in areas ofthe device except areas under the conductive contacts. The removal ofcap layer may be partial (resulting in a cap layer thickness that isless than the initial thickness) or complete (i.e. no cap layer remainsexcept under the deposited contacts). Additionally, in some embodiments,the cap layer thickness may vary from location to location. Typically, acap layer thickness of 500 Angstroms or less will permit a significantpercentage of tritium betas to pass through.

In another embodiment, a thin cap layer (for example 50-500 Angstroms orless) may be grown, and a conductive contact(s) (e.g. metalcontacts/gridlines/ring/terminals/pads/points/epoxy(s)/solders etc.) maybe placed or deposited on the thin cap layer. This cap layer may bechemically etched, physically ablated, or otherwise removed on areas ofthe device except areas under the conductive contact(s). The removal ofcap-layer may be partial (resulting in a cap layer thickness that isless than the initial thickness) or complete (i.e. no cap layer remainsexcept under the deposited contacts) or its thickness may vary fromlocation to location.

In another embodiment, the cap layer is not removed and is insteadinitially grown to the desired thickness prior to deposition of theconductive contact(s).

In yet another embodiment, a cap layer is not grown initially, or isentirely removed prior to deposition of conductive contact(s). In thisembodiment, electrical contact is established with either the window oremitter layer using methods known to those skilled in the art.

In solar cell operation the GaAs cap layer is typically removed exceptunder the metal gridline contacts. This is required since a cap layeracross regions between the metal gridline contacts reduces theefficiency of the solar cell due to significant absorption of the solarphotons. For this reason, the GaAs cap layer is etched away completelyin a solar cell, except for the regions under the gridline metalcontacts. Since in solar cell operation remaining segments of the GaAscap layer under the metal gridlines serve as a conduit for electrons toreach the gridlines, the GaAs cap layer is normally doped to a highlevel (for example, 10̂19 ND/cm̂3). This high doping level provides a goodconductive path for current flow (i.e., in the range of milliamps orhigher for good photovoltaic operation).

High doping of the GaAs cap may unfortunately create defects in then-type InAIP window layer, which could increase the dark current. Forbetavoltaic operations, such a high doping level, the attendant defects,and the resulting increase in the dark current may reduce the overallefficiency of the betavoltaic cell. This is not important forphotovoltaic operations since the dark current is so low compared to themilliamp current levels generated in a solar cell photovoltaic system,but it is extremely important for efficient betavoltaic operation, wherethe generated current levels are in the range of nanoamps. For thisreason, the novel application of a cap layer with reduced doping may beused. Therefore, betavoltaic GaAs cap layer doping may be reduced to alevel of 10̂18 ND/cm̂3, or less, thereby reducing the number of defectsthat may result from diffusion of the GaAs cap layer dopants into then-type InAlP window layer.

As known by those skilled in the art, generally when referring to dopantlevels herein the optimum dopant level is typically a function of thematerial receiving the dopants and the dopant material. Dopant levelsset forth herein are therefore merely exemplary, as other dopant levelsmay also result in a functional device, again, depending on the dopantmaterial and the material receiving the dopants.

The novel intrinsic InGaP layer (between the base and emitter in FIG. 2)is not used in photovoltaic operation, but may be important forbetavoltaic operation due to the fact that it helps achieve low darkcurrents. All layers of the device are grown at high temperatures (e.g.500° C.-700° C.). In particular, the intrinsic layer, the n-InGaPemitter layer, the n-InAlP window layer and the n-GaAs cap layer are allgrown at high temperatures. During the time required for growth of theselayers, the p-type dopant zinc in the p-InGaP layer (base) will diffusetoward the n-type films. If the intrinsic layer is too thin and therebyallows zinc to diffuse into the emitter layer and the InAIP windowlayer, the dark current will increase and the betavoltaic deviceperformance degraded. Thus, since low dark currents are critical fortritium betavoltaic energy conversion, the intrinsic layer must be thickenough to be an effective buffer to zinc diffusion. An intrinsic layerof approximately 1000-3000 Angstroms or more is sufficient to produce alow dark current in a betavoltaic device. However, in certainembodiments it is possible to remove the intrinsic layer or to use asubstantially thinner intrinsic layer thickness of about e.g., 50-100Angstroms.

In certain embodiments described herein, an intrinsic layer is disposedbetween the emitter (e.g., the nInGaP layer) and base (e.g., the pInGaPlayer). See FIG. 2. The thickness of the intrinsic layer is selected sothat most of the beta particle absorption occurs in the emitter andintrinsic layers.

In general, the intrinsic layer in tritium betavoltaic devices servesthree important purposes: (a) it acts as a buffer to diffusion of dopantatoms from the base region into the emitter region; (b) it allowsefficient collection of electron-hole pairs produced as a result of betaparticle absorption; and (c) as a consequence, the base region can beheavily doped so that the built-in junction voltage can be maximized.The high dopant density in the base region (with reference to FIG. 2,the pInGaP base layer) is novel to the betavoltaic structure. This isdue to the fact that it is not necessary to have a finite diffusionlength in the base region for efficient carrier collection; hence arelatively high dopant density can be used in the base region tomaximize the built-in potential. Minimizing diffusion of dopant atomsfrom the base to the emitter and window layers is desirable forachieving a low dark current. With EHPs mainly produced in the emitterand high-field intrinsic region, a large collection efficiency can beachieved.

Homojunctions are typically formed by abruptly reducing one dopant(e.g., for n-type material) and immediately introducing the other dopant(e.g., for p-type material). The intrinsic layer formed in devicesdiscussed herein is created by reducing one dopant input to zero,followed by film growth with neither donors nor acceptors to form theintrinsic layer, and then introducing the other dopant type.

However, since the tritium betas are absorbed in a few thousandAngstroms, there is substantial flexibility regarding an increaseddoping density in the base. As the dopant density is increased in thebase there is the risk of creating defects, but benefiting from a highfield created across the intrinsic/emitter region, thereby raising theoutput voltage of the device.

It should be noted that the tritium InGaP betavoltaic structuresdescribed herein present novel and non-obvious features that provide alow dark current at a high voltage and collection efficiency. Thefollowing data was obtained with solid tritiated metal hydride sources(e.g. titanium tritide, scandium tritide, etc.) and one of the highestreported efficiencies of about 7.5% with respect to the incident betaradiation impinging on the InGaP homojunction. In particular, for atritiated scandium source with a 250 to 500 nanometer thick scandiumfilm and an InGaP homojunction as shown in FIG. 2, the following resultswere achieved:

-   -   Short Circuit Current=45.2 nA/cm̂2    -   Open Circuit Voltage=0.77    -   Fill Factor=0.79,    -   Power=27.5 nW/cm̂2    -   Tritiated Scandium Source=0.369 microwatts/cm̂2,    -   Efficiency=7.5%

In yet another embodiment the dopants may be reversed for all layers toproduce a p/n homojunction structure. In particular, starting from thebottom:

-   -   an nGaAs substrate    -   an nGaAs layer (grown to establish a crystal structure)    -   an n+InGaP layer (a back-surface field reflector or minority        carrier reflector layer)    -   an nInGaP base layer    -   an intrinsic InGaP layer (for preventing diffusion of dopants        between the p-doped and n-doped layers)    -   a pInGaP emitter layer    -   a pInAlP window layer (preferably having a lattice structure        closely matched to the lattice structure of the overlaying        pInGaP emitter layer    -   that allows holes to pass to the cap layer and reflects        electrons back to the emitter)    -   a pGaAs cap layer (may be highly doped p-type)

Tritium beta particle penetration in semiconductors is less than aboutone micron. Thus, it is clear that the emitter and window layers need tobe very thin, preferably on the order of a few hundred Angstroms, sothat most of the beta particle absorption occurs in the high electricfield region of the depletion layer (with respect to FIG. 2, the highelectric field region in the intrinsic InGaP layer or in an embodimentabsent the intrinsic layer, the high field region between the p-dopedbase and an n-doped emitter).

In another embodiment, the InGaP betavoltaic structure may be alloyedwith 1-3% aluminum to achieve a slightly higher band gap, resulting in ahigher open circuit voltage. This structure may result in a higherefficiency. But efficiency must also consider material defects, theideality factor (indicative of the extent to which performance of abetavoltaic device approaches an ideal diode), and form factor.

Without loss of generality, the various embodiments described for anInGaP betavoltaic structure (and more generally, all radioisotope directenergy conversion structures) discussed herein may be extended tobetavoltaic structures that include InAlP, InAlGaP, GaAs, AlGaAs, andother Group III-V compounds. Any of these materials may belattice-matched to GaAs and germanium (Ge) substrates, either by directcontact to the substrate or by use of an intervening lattice-matchinglayer.

The various structures described herein for use with InGaP betavoltaicjunction materials (e.g., the cap layer, the window layer, theback-surface filed reflector layer, the intrinsic layer) are alsorelevant to semiconductor structures of InAlP, InAlGaP, GaAs, AlGaAs,and one or more other Group III-V materials. Certain of these materialsmay have a higher band gap than InGaP.

As shown in one or more of the tables below, in certain betavoltaicstructures one or more layers may comprise InGaP material (the baselayer for example) and one or more other layers in the structure maycomprise InAlP material (the reflector layer, for example).

FIG. 4 generalizes some of the novel features of the invention. Eachlayer is lattice matched to the substrate so that the number ofdislocations generated by growth of the individual layers is minimized.Generally, in a lattice-matched structure, one or more latticeparameters (e.g., the atomic distance between atoms) may not be exactlyidentical between structure layers, but generally are within a certaintolerance that is acceptable for favorable growth and desirableoperation of the device.

In general, beta particles radiate outward randomly in all directionsfrom the source. The beta particles in FIG. 4 are released from thetritiated metal hydride source in all direction; the arrowheadsrepresent the direction of motion of those beta particles that strikethe homojunction.

FIG. 4 illustrates individual layers of a generalized betavoltaichomojunction semiconductor 30 comprising, from the bottom:

a rear contact/conductor/back metal

a doped substrate (e.g., GaAs or Ge)

a nucleation layer (utilized with certain material layers to providelattice matching)

a crystallization layer (also utilized with certain material layers toprovide lattice matching)

a buffer layer (a preparatory layer for growth of the overlying layers)

a BSFR layer (a back-surface field reflector or minority carrierreflector/majority carrier transmitter layer; in the embodiment of FIG.4 the n-type BSFR reflects minority carrier holes back toward thebuilt-in field between the base and emitter)

a base layer (represents the “bottom” layer of the junction that createsthe built-in field and the depletion region)

an intrinsic layer (prevents diffusion of dopants between the oppositelydoped layers, but permits the built-in field to be established)

an emitter layer (represents the “top” layer of the junction thatcreates the built-in field and the depletion region)

a window layer (closely lattice-matched to the emitter layer below andthe cap layer above, allowing majority carriers to pass to the cap layerand reflecting minority carriers back toward the emitter; in theillustrated embodiment the p-type window layer reflects minority carrierelectrons)

a cap layer (highly doped in one embodiment)

If the dopant types are reversed from those set forth in FIG. 4, thestructure is referred to as a n/p homojunction semiconductor.

FIG. 4 also indicates the electric field (the built-in field) as createdacross the depletion layer.

In the FIG. 4 embodiment the cap layer is present only below conductors32, and not below the beta source.

In the embodiment of FIG. 5 the cap layer is present below the betasource and connected to a conductor 34. A contact 35 is connected to thebuffer layer as shown. Note that both contacts 34 and 35 are present ona same surface of the device, in FIG. 5 referred to as the “Top ofDevice.” Presenting two contacts (with current generated between them)on the same surface of the device is advantageous when connecting suchdevices in a serial or parallel configuration. See the commonly-ownedpatent application entitled Series and/or Parallel Connected Alpha, Betaand Gamma Voltaic Cell Devices, filed on May 22, 2017, and assignedapplication Ser. No. 15/602,078 (Attorney Docket Number 11432-010). Incertain embodiments comprising multiple connected betavoltaic devices,it may also be advantageous to have a rear contact/conductor/backcontact 36.

There are several features of this FIG. 4 structure that allow efficientbetavoltaic energy conversion. These features also apply to otherembodiments of the invention as described herein, (e.g., FIGS. 2 and 5).

(a) High quality, large band gap semiconductor junction (e.g. InGaP,InAlGaP, InAlP) resulting in a highly efficient device;

(b) Back-surface field reflector that reflects minority carriers backonto the junction field. It may comprise a higher-band-gaplattice-matched layer, or a highly-doped layer of similar composition tothe base, a pseudomorphic layer, or layers comprising other materialsand techniques known in the art);

(c) A window layer that reflects minority carriers back to the emitterleading to a low dark current. This layer may comprise a higher-band-gaplattice-matched layer, or a highly-doped layer of similar composition tothe emitter, a pseudomorphic layer, or layers comprising other materialsand techniques known in the art);

(d) A cap layer covering the top surface and sufficiently thin to permitthe transmission of beta particles into the betavoltaic homojunctionstructure;

(e) a 1000 to 3000 Angstrom intrinsic layer to act as a buffer to thediffusion of the base dopant (usually Zn) into the emitter region.

The features (a), (b) and (c) above may be important for solar celloperation, but their utilization in tritium betavoltaic application isconsidered novel in the present embodiment. The novel features (d) and(e) may be important for betavoltaic conversion, but they are notnecessarily used for photovoltaic energy conversion.

It should be noted that the tritium betavoltaic structure presents noveland non-obvious features that provide a low dark current and a highvoltage and collection efficiency. For example, the InAlP structure asdescribed in Table 3 and Table 6 can yield up to an open circuit voltagebetween 1.0 and 1.2 Volts, a fill factor of 0.8 and an efficiency >11.5%based on the incident beta flux impinging on the semiconductor.

It should also be noted that several betavoltaic structure embodimentsutilize tunnel junctions to serve as a means of changing the dominantcarrier from electrons to holes, or vice versa. For example, tunneljunction concepts are employed in an embodiment with an n/p junction ona p-type substrate or in an embodiment with a p/n junction on an n-typesubstrate.

The tunnel junction is used to change the carriers and allow use asubstrate doped the opposite type as the junction's base. Two differenttypes of layers are utilized in the tunnel junction structures. Thesetunnel junctions involve a heavily doped n-layer adjacent to a heavilydoped p-layer, referred to as n⁺⁺ and p⁺⁺ layers, respectively.Thicknesses are typically 100 Angstroms for both the n⁺⁺ and p⁺⁺ layersalthough they can range from approximately 50 Angstroms to 200Angstroms, but can be made thinner or thicker in certain embodiments.The dopant levels are typically 5E18 to 1E19 cm-3 for Zinc in p⁺⁺ layersand similarly for Silicon doping in n⁺⁺ layers. However, other dopanttypes and concentrations known to those skilled in the art may be used.

Various betavoltaic structures based on GaAs and Ge substrates aresummarized in Tables 1-6 below. It should be noted that Tables 1-6 areexemplary in nature and that other structures, layers, compositions,dopant types, dopant concentrations, and stoichiometries may beutilized. Other features of various materials and layers are set forthin the reference notes provided with the Tables.

TABLE 1 InGaP Cells on GaAs Substrates p-GaAs Substrates n-GaAsSubstrates n/p InGaP p/n InGaP n/p InGaP p/n InGaP Layer 1 pGaAs(substrate) pGaAs (substrate) nGaAs (substrate) nGaAs (substrate) Layer2 pGaAs (Est Crs Struc) pGaAs (Est Crs nGaAs (Est Crs Struc) nGaAs (EstCrs Struc) Struc) Layer 3 pInAlP (reflector) p++GaAs (p-layer TJ)n++GaAs (p-layer TJ) nInAlP (reflector) Layer 4 pInGaP (base) n++GaAs(n-layer TJ) p++GaAs (n-layer TJ) nInGaP (base) Layer 5 InGaP (i-Layer)nInAlP (reflector) pInAlP (reflector) InGaP (i-Layer) Layer 6 nInGaP(emitter) nInGaP (base) pInGaP (base) pInGaP (emitter) Layer 7 nInAlP(window) InGaP (i-layer) InGaP (i-layer) pInAlP (window) Layer 8 n++GaAs(cap Layer) pInGaP (emitter) nInGaP (emitter) p++GaAs (cap Layer) Layer9 pInAlP (window) nInAlP (window) Layer 10 p++GaAs (cap layer) n++GaAs(cap layer) Notes: 1. Est Crs Struc designates a layer for establishingthe crystal structure. 2. TJ designates a tunnel junction. 3. InGaPrefers to a compound In_(x)Ga_((1−x))P, (e.g. where x = 0.485 ± 0.01),or another appropriate stoichiometry that lattice matches GaAs known tothose skilled in the art. 4. InAlP refers to a compoundIn_(x)Al_((1−x)), (e.g. where x = 0.48 ± 0.02), or another appropriatestoichiometry that lattice matches GaAs known to those skilled in theart. 5. P-type materials will typically be doped with Zn or otherdopants known in the art with the dopant concentrations in the range ofE16 cm−3 to E19 cm−3. It should be noted that the p, p+, and p++designations represent successive higher dopant concentrations. 6.N-type materials are doped with Si or Te or other dopants known in theart with the doping density in the range of about E16 cm−3 to E19 cm−3.It should be noted that the n, n+, and n++ designations representsuccessively higher concentration ranges. 7. Reflector layer refers tolayer at an interface between the base layer and the substrate thatreflects minority carriers to minimize recombination losses. 8. Windowlayer refers to a layer adjacent the emitter that allows majoritycarriers pass to the cap layer and reflects minority carriers tominimize recombination losses. 9. The cap layer is generally heavilydoped so that it is very conductive. 10. Cap layer dopant concentrationsn++ and p++ may be changed to n or n+ and p or p+ respectively, incertain embodiments.

TABLE 2 In(AlGa)P Cells on GaAs Substrates p-GaAs Substrates n-GaAsSubstrates n/p In(AlGa)P p/n (AlGa)P n/p (AlGa)P p/n (AlGa)P Layer 1pGaAs (substrate) pGaAs (substrate) nGaAs (substrate) nGaAs (substrate)Layer 2 pGaAs (Est Crs Struc) pGaAs (Est Crs nGaAs (Est Crs Struc) nGaAs(Est Crs Struc) Struc) Layer 3 pInAlP (reflector) p++GaAs (p-layer TJ)n++GaAs (p-layer TJ) nInAlP (reflector) Layer 4 pIn(AlGa)P (base)n++GaAs (n-layer TJ) p++GaAs (n-layer TJ) nIn(AlGa)P (base) Layer 5In(AlGa)P (i-layer) nInAlP (reflector) pInAlP (reflector) In(AlGa)P(i-layer) Layer 6 nIn(AlGa)P (emitter) nIn(AlGa)P (base) pIn(AlGa)P(base) pIn(AlGa)P (emitter) Layer 7 nInAlP (window) In(AlGa)P (i-layer)In(AlGa)P (i-layer) pInAlP (window) Layer 8 n++GaAs (cap layer)pIn(AlGa)P (emitter) nIn(AlGa)P (emitter) p++GaAs (cap layer) Layer 9pInAlP (window) nInAlP (window) Layer 10 p++GaAs (cap layer) n++GaAs(cap layer) Notes: 1. Est Crs Struc designates a layer to establish thecrystal structure. 2. TJ designates a tunnel junction. 3. InAlP refersto a compound In_(x)Al_((1−x)), (e.g. where x = 0.48 ± 0.02), or anyother appropriate stoichiometry that lattice matches GaAs known to thoseskilled in the art. 4. In(AlGa)P refers a compoundIn_(x)(Al_(y)Ga_((1−y)))_((1−x))P (e.g. where x = 0.48 ± 0.01 and y =0.2 ± 0.1), or any other appropriate stoichiometry that lattice matchesGaAs known to those skilled in the art. 5. P-type materials willtypically be doped with Zn or other dopants known in the art with thedopant concentrations in the range of about E16 cm−3 to E19 cm−3. Itshould be noted that the p, p+, and p++ designations representsuccessively higher dopant concentrations. 6. N-type materials are dopedwith Si or Te or other dopants known in the art with the doping being inthe range of about E16 cm−3 to E19 cm−3. It should be noted that the n,n+, and n++ designations represent successive higher concentrationranges. 7. A reflector layer refers to layer at an interface between thebase layer and substrate that reflects minority carriers so thatrecombination losses are minimized. 8. A window layer refers to a layeradjacent the emitter that allows majority carriers pass to the cap layerand reflects minority carriers to minimize recombination losses. 9. Thecap layer is heavily doped so that it is very conductive. 10. Cap layerdopant concentrations n++ and p++ may be interchanged with n or n+ and por p+, respectively.

TABLE 3 InAlP Cells on GaAs Substrates p-GaAs Substrates n-GaAsSubstrates n/p InAlP p/n InAlP n/p InAlP p/n InAlP Layer 1 pGaAs(substrate) pGaAs (substrate) nGaAs (substrate) nGaAs (substrate) Layer2 pGaAs (Est Crs Struc) pGaAs (Est Crs nGaAs (Est Crs Struc) nGaAs (EstCrs Struc) Struc) Layer 3 p+ InAlP (reflector) p++GaAs (p-layer TJ)n++GaAs (p-layer TJ) n+InAlP (reflector) Layer 4 pInAlP (base) n++GaAs(n-layer TJ) p++GaAs (n-layer TJ) nInAlP (base) Layer 5 InAlP (i-layer)n+InAlP (reflector) p+InAlP (reflector) InAlP (i-layer) Layer 6 nInAlP(emitter) nInAlP (base) pInAlP (base) pInAlP (emitter) Layer 7 n+InAlP(window) InAlP (i-layer) InAlP (i-layer) p+InAlP (window) Layer 8 n+GaAs(cap Layer) pInAlP (emitter) nInAlP (emitter) p+GaAs (cap Layer) Layer 9p+InAlP (window) n+InAlP (window) Layer 10 p++GaAs (cap layer) n++GaAs(cap layer) Notes: 1. Est Crs Struc designates a layer that establishesa crystal structure. 2. TJ designates a tunnel junction. 3. InAlP refersto a compound In_(x)Al(_(1−x)), (e.g. where x = 0.48 ± 0.02), or anyother appropriate stoichiometry that lattice matches GaAs known to thoseskilled in the art. 4. P-type materials are typically doped with Zn orother dopants known in the art, with the dopant concentrations being inthe approximate range of E16 cm−3 to E19 cm−3. It should be noted thatthe p, p+, and p++ designations represent successively higher dopantconcentrations. 5. N-type materials are doped with Si or Te or otherdopants known in the art, with the doping concentration in the range ofabout E16 cm−3 to E19 cm−3. It should be noted that the n, n+, and n++designations represent successively higher concentration ranges. 6. Areflector refers to layer at an interface between the base layer andsubstrate that reflects minority carriers so that recombination lossesare minimized. 7. A window refers to a layer adjacent the emitter thatallows majority carriers to pass to the cap layer and reflects minoritycarriers to minimize recombination losses. 8. The cap layer is heavilydoped so that it is very conductive. 9. Cap layer n-type and p-typedopant concentrations may be respectively selected as n, n+, n++ and p,p+, p++.

In other embodiments the GaAs substrate of Tables 1-3 is replaced by agermanium substrate. The overlying semiconductor layers may comprise Geor GaAs. Growth of high quality GaAs layers on germanium requires growthof a nucleation layer on the germanium to create a virtual GaAssubstrate. One approach grows a first GaAs layer at a relatively lowtemperature of about 550 degrees C. followed by growth of second GaAslayer at a temperature more commonly used for GaAs materials, namelyabout 700 degrees C. Growth of the low temperature GaAs layer leads to arelatively smooth GaAs layer, which improves subsequent growth of thehigh temperature GaAs layer.

In one embodiment a deliberate impurity (e.g. Indium) may be introducedduring the nucleation growth process, which has the effect of replacingatoms in the crystal structure to change certain lattice parameters.This technique is implemented to permit the matching of latticeparameters for chemical systems that would normally have very differentlattice constants, even when they have the same crystal structure

To reduce the generation of anti-phase boundaries, which can lead torecombination centers, off-oriented Ge materials (such as Ge(001)) maybe used for growth of the low temperature GaAs layer. Althoughmodifications of this approach have been developed, growth of GaAsnucleation layers, or other nucleation layers intended for use with thestructures outlined in other embodiments, resulting in high quality GaAsfilms on Ge substrates usually involve these two features: use ofoff-oriented Ge(001) substrates and the low temperature film of GaAs.

In general, layers in betavoltaic structures based on Ge substratesparallel those grown on GaAs substrates. Both n- and p-type Gesubstrates are used and both must first have nucleation layers grown toachieve the growth of high-quality GaAs films.

However, one unique feature must be dealt with when growingsemiconductor layers on p-Ge substrates. Growth of As-containing andP-containing films on a p-Ge substrate results in the formation of ann-type layer on the surface of the Ge substrate. To counter theformation of this artifact layer, the first layer grown on the p-type Gesubstrate is heavily doped p-GaAs. This GaAs layer thus serves twopurposes, it establishes a GaAs crystalline structure and the large Zndoping level (about 1E18 to about 5E18) mitigates the potential problempresented by the possible formation of an n-type artifact layer on thep-type Ge substrate, in both cases of n- and p-type Ge substrates.

Generally, the references to a nucleation layer or a nucleation processherein refer to the growth of layers identified as a nucleation layerand a crystallization layer. A buffer layer may also be formed abovethese two layers. See, for example, FIG. 4.

Structures based on InGaP, In(AlGa)P and InAlP grown on Ge substratesare exemplified by, but not limited to, Tables 4, 5 and 6.

TABLE 4 InGaP Cells on Ge Substrates p-Ge Substrates n-Ge Substrates n/pInGaP p/n InGaP n/p InGaP p/n InGaP Layer 1 pGe (substrate) pGe(substrate) nGe (substrate) nGe (substrate) Layer 2 p+GaAs (Nucl Layer)p+GaAs (Nucl Layer) nGaAs (Nucl Layer) nGaAs (Nucl Layer) Layer 3 p+GaAs(Est Crs p+GaAs (Est Crs nGaAs (Est Crs nGaAs (Est Crs Struc) Struc)Struc) Struc) Layer 4 pInAlP (reflector) p++GaAs (p-layer TJ) n++GaAs(n-layer nInAlP (reflector) TJ) Layer 5 pInGaP (base) n++GaAs (n-layerTJ) p++GaAs (p-layer TJ nInGaP (base) Layer 6 InGaP (i-layer) nInAlP(reflector) pInAlP (reflector) InGaP (i-layer) Layer 7 nInGaP (emitter)nInGaP (base) pInGaP (base) pInGaP (emitter) Layer 8 nInAlP (window)InGaP (i-layer) InGaP (i-layer) pInAlP (window) Layer 9 n+GaAs (capLayer) pInGaP (emitter) nInGaP (emitter) p+GaAs (cap Layer) dLayerpInAlP (window) nInAlP (window) 10 Layer 11 p+GaAs (cap layer) n+GaAs(cap layer) Notes: 1. Est Crs Struc designates a layer for establishingthe crystal structure. 2. Nucl Layer designates the nucleation layer asdiscussed elsewhere herein. 3. Layer 3 for the structures grown on p-Gesubstrates is doped with Zn to a level of about 1E18 cm−3 to preventformation of an artifact n-layer. The layer also establishes the GaAscrystal structure. For the p-Ge substrate and the n/p InGaP embodimentit may not be necessary to dope layer 3 (see the Table above) to a p+level, as a lower doping may be sufficient to establish the crystalstructure without causing formation of an artifact layer at theinterface between the two, and subsequently needing to suppress thecreation of this artifact layer. As descried elsewhere herein, theartifact layer can be caused by mixing of the Ge and InGaP at theboundary. 4. TJ designates a tunnel junction 5. InGaP refers to acompound In_(x)Ga_((1−x))P, (e.g. where x = 0.485 ± 0.01), or any otherappropriate stoichiometry that lattice matches GaAs, as known to thoseskilled in the art. 6. InAlP refers to a compound In_(x)Al_((1−x)),(e.g. where x = 0.48 ± 0.02), or any other appropriate stoichiometrythat lattice matches GaAs known to those skilled in the art. 7. P-typematerials will typically be doped with Zn or other dopants known in theart with the dopant concentrations being in the range of E16 cm−3 to E19cm−3. It should be noted that the p, p+, and p++ designations representsuccessively higher dopant concentrations. 8. N-type materials are dopedwith Si or Te or other dopants known in the art with the doping being inthe range of about E16 cm−3 to E19 cm−3. It should be noted that the n,n+, and n++ designations represent successively higher concentrationranges. 9. A reflector layer refers to layer at an interface between thebase layer and substrate that reflects minority carriers so thatrecombination losses are minimized. 10. A window layer refers to a layeradjacent the emitter that allows majority carriers to pass to the caplayer and reflects minority carriers to minimize recombinations. 11. Thecap layer is heavily doped so that it is very conductive as n+ for then-type window or p+ for the p-type window. 12. The cap layer may also ben or n++ or p or p++

TABLE 5 In(AlGa)P Cells on Ge Substrates p-Ge Substrates n-Ge Substratesn/p In(AlGa)P p/n In(AlGa)P n/p In(AlGa)P p/n In(AlGa)P Layer 1 pGe(substrate) pGe (substrate) nGe (substrate) nGe (substrate) Layer 2p+GaAs (Nucl Layer) p+GaAs (Nucl Layer) nGaAs (Nucl Layer) nGaAs (NuclLayer) Layer 3 pGaAs (Est Crs Struc) pGaAs (Est Crs nGaAs (Est Crs nGaAsEst Crs Struc) Struc) Struc) Layer 4 pInAlP (reflector) p++GaAs (p-layern++GaAs (n-layer nInAlP (reflector) TJ) TJ) Layer 5 pIn(AlGa)P (base)n++GaAs (n-layer p++GaAs (p-layer nIn(AlGa)P (base) TJ) TJ) Layer 6In(AlGa)P (i-layer) nInAlP (reflector) pInAlP (reflector) In(AlGa)P(i-Layer) Layer 7 nIn(AlGa)P (emitter) nIn(AlGa)P (base) pIn(AlGa)P(base) pIn(AlGa)P (emitter) Layer 8 nInAlP (window) In(AlGa)P (i-layer)In(AlGa)P (i-layer) pInAlP (window) Layer 9 n+GaAs (cap Layer)pIn(AlGa)P (emitter) nIn(AlGa)P (emitter) p+GaAs (cap Layer) Layer 10pInAlP (window) nInAlP (window) Layer 11 p+GaAs (cap layer) n+GaAs (caplayer) Notes: 1. Est Crs Struc designates a layer to establish thecrystal structure 2. Nucl Layer designates a nucleation layer asdiscussed elsewhere herein. 3. Layer 3 for the structures grown on p-Gesubstrates is doped with Zn to a level of about 1E18 cm−3 to preventformation of the artifact n-layer. The layer also establishes GaAsstructure. 4. TJ designates a tunnel junction 5. InAlP refers to acompound In_(x)Al_((1−x)), (e.g. where x = 0.48 ± 0.02), or any otherappropriate stoichiometry that lattice matches GaAs known to thoseskilled in the art. 6. In(AlGa)P refers a compoundIn_(x)(Al_(y)Ga_((1−y)))_((1−x))P (e.g. where x = 0.48 ± 0.01 and y =0.2 ± 0.1), or any other appropriate stoichiometry that lattice matchesGaAs known to those skilled in the art. 7. P-type materials willtypically be doped with Zn or other dopants known in the art with thedopant concentrations being in the range of E16 cm−3 to E19 cm−3. Itshould be noted that the p, p+, and p++ designations representsuccessively higher dopant concentrations. 8. N-type materials are dopedwith Si or Te or other dopants known in the art with the doping being inthe range of about E16 cm−3 to E19 cm−3. It should be noted that the n,n+, and n++ designations represent successively higher concentrationranges. 9. A reflector layer refers to layer at an interface between thebase layer and a substrate that reflects minority carriers so thatrecombination losses are minimized. 10. A window layer refers to a layeradjacent the emitter that allows majority carriers pass to the cap layerand reflects minority carriers to minimize recombination losses. 11. Thecap layer is heavily doped so that it is very conductive as n+ for then-type window or p+ for the p-type window. 12. The cap layer may also ben or n++ or p or p++

TABLE 6 InAlP Cells on Ge Substrates p-Ge Substrates n-Ge Substrates n/pInAlP p/n InAlP n/p InAlP p/n InAlP Layer 1 pGe (substrate) pGe(substrate) nGe (substrate) nGe (substrate) Layer 2 p+GaAs (Nucl Layer)p+GaAs (Nucl Layer) nGaAs (Nucl Layer) nGaAs (Nucl Layer) Layer 3 p+GaAs(Est Crs p+GaAs (Est Crs Struc) nGaAs (Est Crs nGaAs (Est Crs Struc)Struc) Struc) Layer 4 p+InAlP (reflector) p++GaAs (p-layer TJ) n++GaAs(p-layer n+ InAlP (reflector) TJ) Layer 5 pInAlP (base) n++GaAs (n-layerTJ) p++GaAs (n-layer nInAlP (base) TJ) Layer 6 InAlP (i-layer) n+InAlP(reflector) p+InAlP (reflector) InAlP (i-layer) Layer 7 nInAlP (emitter)nInAlP (base) pInAlP (base) pInAlP (emitter) Layer 8 n+InAlP (window)InAlP (i-layer) InAlP (i-layer) p+InAlP (window) Layer 9 n+GaAs (capLayer) pInAlP (emitter) nInAlP (emitter) p+GaAs (cap Layer) Layer 10p+InAlP (window) n+InAlP (window) Layer 11 p+GaAs (cap layer) n+GaAs(cap layer) Notes: 1. Est Crs Struc designates to a layer to establishcrystal structure 2. Nucl Layer designates a nucleation layer asdiscussed elsewhere herein. 3. Layer 3 for the structures grown on p-Gesubstrates is doped with Zn to a level of about 1E18 cm−3 to preventformation of an artifact n-layer. The layer also establishes GaAsstructure. 4. TJ designates a tunnel junction 5. InAlP refers to acompound In_(x)Al_((1−x)), (e.g. where x = 0.48 ± 0.02), or any otherappropriate stoichiometry that lattice matches GaAs known to thoseskilled in the art. 6. P-type materials will typically be doped with Znor other dopants known in the art with the dopant concentrations beingin the range of E16 cm−3 to E19 cm−3. It should be noted that the p, p+,and p++ designations represent successive higher dopant concentrations.7. N-type materials are doped with Si or Te or other dopants known inthe art with the doping being in the range of about E16 cm−3 to E19cm−3. It should be noted that the n, n+, and n++ designations representsuccessive higher concentration ranges. 8. Reflector refers to layer atan interface between the base layer and the substrate that reflectsminority carriers so that recombination losses are minimized. 9. Awindow refers to a layer adjacent the emitter that allows majoritycarriers pass to the cap layer and reflects minority carriers tominimize recombination losses. 10. The cap layer is heavily doped sothat it is very conductive as n+ for the n-type window or p+ for thep-type window. 12. The cap layer may also be n or n++ or p or p++.

It should be noted that in one embodiment a betavoltaic cell may begrown on each side of the semiconductor substrate. For instance, foreither a GaAs or a Ge substrate, a combination of a p/n and n/pbetavoltaic structure (e.g. InGaP, InAlP, InAlGaP, or others) may begrown on opposing sides of a common substrate to create a bilateralbetavoltaic structure (i.e., a double-sided betavoltaic structure)yielding twice the open circuit voltage of a single-sided counterpart.Just as in the case of a single-sided betavoltaic, each side of thebilateral cell requires a source of beta flux impinging on therespective junctions. The use of tunnel junctions may be required toallow the current to flow in a series arrangement between the twobetavoltaic cells.

In one embodiment of the present invention, the tritium source is atritium metal hydride (sometimes referred to as a metal tritide), incontact with the top surface of the betavoltaic structure as shown inFIG. 1 or more generally as a beta source in FIG. 4. The metal tritidemay be formed by depositing one or a combination of hydride formingmetals (e.g. scandium, titanium, magnesium, palladium, lithium etc.). Athickness of the metal tritide layer is typically less than one micron,but can be thicker. The metal tritide layer may be as thin as 50-100nanometers, but in certain embodiments may be thinner.

The metal layer may be placed on top of the betavoltaic cell by directlydepositing on top of the betavoltaic cell's active area (e.g. window orcap layer) through methods known in the art (e.g. evaporation, electrodeposition etc.).

The tritium metal hydride may comprise a combination of metals, metallayers, or alloys capable of absorbing or retaining tritium in its metalmatrix. Alternatively, the metal tritide layer may be deposited on aseparate thin substrate (e.g. ˜25 microns or less to ˜500 microns or upto millimeter thickness range) that is mechanically connected to thebetavoltaic cell's active area via a pressure joint, or by using epoxyspot welding.

The metal tritide is typically formed by exposure to tritium gas atpressures ranging 0.25 to 20 Bar and temperatures ranging approximately100° C. to 600° C. for durations ranging minutes to days. It should benoted that metal tritides can also be formed with temperatures andpressures outside of the above-mentioned range and can also be formedthrough chemical and electrochemical reactions as is known in the art.

A layer of palladium ranging from approximately 1 nanometer to 500nanometers may be deposited over (i.e. capping-off) a scandium,titanium, magnesium or lithium metal or other tritide forming metal,combination of metals, metal layers or alloys in order to reduce thetritium loading temperature and stabilize the tritium within the metalmatrix after the tritide has been formed. The metal tritide layer mayalso be formed by an in-situ evaporation of the metal in the presence oftritium.

Bi-directional metal tritide sources (i.e. with betas emanating fromopposing surfaces) may be utilized in one embodiment of this invention.For example, the metal tritide may be formed as a film on top of thebetavoltaic cell's active semiconductor area such that a first surfaceof the metal tritide is in contact with or proximate to thesemiconductor area. Then a second cell can be placed in direct contactwith (or proximate to) a second surface of the metal tritide. See inparticular FIGS. 7B (and its associated FIG. 7A) and 8B (and itsassociated FIG. 8A).

In an embodiment of FIG. 15, a betavoltaic junction device comprises, abeta source 50, a stack of n and p semiconductor layers 52, a betasource 60, and a stack of n and p semiconductor layers 62 oriented asshown. Beta particles generated by the beta source 50 pass into thesemiconductor layers 52 and 62, and beta particles emanating from thebeta source 60, and are also absorbed in the semiconductor layers 52 and62.

In another embodiment of the present invention the conductive contactlines on the top surface of the betavoltaic homojunction can be verythin and extend along the perimeter of the semiconductor. Such a contactring collects the current from the semiconductor while providing aminimal shadowing effect to the radioactive source's beta flux thatimpinges on the surface of the semiconductor. The contact ring for thebetavoltaic semiconductor may be formed in the same manner as solar cellindustry uses to make contact gridlines on the solar cell semiconductor.However, the betavoltaic cell contact ring is substantially differentfrom a solar cell where a series of gridlines uniformly cover thesurface of the semiconductor and can cover approximately 5-10% of thesemiconductor surface. This uniform coverage creates a shadowing effectresulting in a proportional loss of power from the solar cell. Incontrast, the betavoltaic cell's contact ring may be reduced to a smallperimeter (e.g. outlining a 1 cm×1 cm cell or 3 cm×3 cm cell etc.) or itmay comprise only a contact point or set of contact points or lines.This configuration may be utilized due to the low magnitude of currentcollected from the betavoltaic device (in the nanoamp to microamp persquare centimeter range), as opposed to solar cells where the range isgenerally in the milliamp per square centimeter range. Thus, whereassolar cells require relatively low series resistance (less than one ohmper square centimeter of cell area) by the inclusion of more contactline coverage, betavoltaic cells can function efficiently with muchgreater values of series resistance due to the small current valuesgenerated by betavoltaic devices.

The conductive contacts on the rear surface (also referred to as thebottom or back surface) of the betavoltaic homojunction device (see FIG.4 or 5, for example) can be formed by many techniques known to thoseskilled in the art. When a conductive substrate is utilized, and ajunction is grown on only one side of the conductive substrate, e.g.,the front side, the rear contact may be established with blanketdeposition of conductive contact materials onto the bottom surface ofthe substrate. Rear or bottom contacts may also be engineered on thebottom side of the substrate to meet specific requirements and may be inthe form of metalcontacts/gridlines/ring/terminals/pads/points/epoxy(s)/solders, etc.Similar to the betavoltaic cell's top contacts, the rear or bottomcontact may be formed as a ring and may be reduced to a small perimeter(e.g. outlining a 1 cm×1 cm cell or 3 cm×3 cm cell etc. may compriseonly contact points or lines.

In an embodiment utilizing a non-conductive or insulating substrate, therear contact can be established at any point where access can be gainedto a material doped to the type opposite to that of the material layerproximate to the front contact. Specifically, in one embodiment thefront contact is established to the cap layer such that the rear contactmay be established with one of the opposite polarity layers (e.g.,buffer, BSF, base, etc.).

FIG. 4 illustrates a case in which a rear contact is presented on a top(or front) surface of the device by contacting the n-type buffer layer.In this illustration, a front contact is established on the top surfaceto the p-type doped cap layer. As can be seen in FIG. 4, a portion ofthe top layers may be chemically etched, physically ablated, orotherwise removed to expose the two oppositely-doped layers forreceiving the respective contacts.

Conversely, a portion of the substrate may be chemically etched,physically ablated, or otherwise removed partially from the bottom toreach these layers. In this case contacts to both the n-type and p-typelayers are presented on the bottom/rear/back surface of the device.

In any case, the exposed region(s) of the semiconductor layers mayaccommodate contacts in the form of metalcontacts/gridlines/ring/terminals/pads/points/epoxy(s)/solders, etc.

It should be noted that any of the approaches described herein may alsobe utilized for conductive substrates.

In any of the embodiments presented herein, device terminals may beintroduced on any surface of the junction device (e.g., homo junctionsor hetero junctions) provided that the layer is physically accessible orcan be accessed through removal of proximal materials by chemicaletching, physical ablation, or otherwise removed by methods known tothose skilled in the art.

In yet another embodiment of this invention, a thin cap layer (e.g.GaAs) is grown to a desired thickness (e.g. 50-500 Angstroms or less)and uniformly covers the betavoltaic window layer. In thisconfiguration, the contact metal gridlines for current collection arereplaced with a tritium metal tritide deposited uniformly over the caplayer. In this configuration, the tritium metal tritide serves as both ametal contact collector and a beta-source emitter resulting in lessshadowing of betas impinging on the betavoltaic cell and a simplerconstruction of the betavoltaic cell. As previously described the GaAscap layer may be replaced by other suitable group III-V materials orcompounds.

The contacts in a betavoltaic semiconductor can result in a shadowcoverage that is much less than about 1%, thereby providing a higherefficiency for the betavoltaic cell (i.e., battery). Specific shadowcoverage and thicknesses of contact ring, lines or dots required by abetavoltaic semiconductor is dictated by consideration of sheetconductance of the top surface cell layers, namely, the cap, window andemitter layers. The sheet resistance for a tritium betavoltaic cell canbe relatively large (e.g. >100 Ohms per square centimeter).

In all embodiments of the present invention it may be desirable toshield the edges of the betavoltaic structure from beta particles. Thisconstitutes another novel aspect of the present invention. As is knownin the art, if the energy of a beta particle is large enough, theparticle can cause the displacement of an atom in a crystallinesemiconductor. Atomic vacancies can act as a recombination center forEHPs in semiconductors and can degrade betavoltaic efficiencies.Fortunately, the threshold for atomic displacement in semiconductors istypically greater than 250 keV. Therefore, tritium beta particles, aswell as beta particles from Promethium-147 and Nickel 63, do not degradesemiconductor diode properties as a result of beta absorption within thebulk of the material. However, low energy betas can create danglingbonds along the junction periphery, which can cause shunting currents orcarrier recombination at the junction edges. If the edges are notproperly shielded or protected from the beta flux, the betavoltaicdevice performance/efficiency may be degraded.

As illustrated in FIGS. 7A, 7B, 8A, and 8B, the junction edges may beprotected by retaining the tritium source within the perimeter contactmetal gridlines at a distance such that the beta particle cannot reachthe edges of the semiconductor. As described previously, the tritidesource may be deposited on the cell's window or cap layer through any ofthe methods known to those skilled in the art or may be present on aseparate substrate or an adjacent betavoltaic cell. Furthermore, themetal perimeter contact gridlines act as a physical barrier to the betaflux, thus preventing the beta particles from striking the edge of thedevice. It should be understood that protection of the edges may beaccomplished through a variety of means such as various forms ofphysical barriers (e.g. deposited metal barriers, polymers, insulators,etc.). According to another technique, ensuring a relatively largephysical distance from the beta particle source to the device edges mayalso suffice.

In all embodiments of the present invention, the voltage and current maybe scaled up via the stacking of betavoltaic semiconductors and tritiumsources (betavoltaic cells). Betavoltaic cell layers may be stackedvertically or arranged horizontally and configured electrically inseries or parallel. See for example, the commonly-owned patentapplication entitled Series and/or Parallel Connected Alpha, Beta andGamma Voltaic Cell Devices, filed on May 22, 2017, and assignedapplication Ser. No. 15/602,078 (Attorney Docket Number 11432-010),which is incorporated herein in its entirety. Electrical connection canbe established by utilizing through-vias as power lead contacts acrossbetavoltaic cell layers, by using current-channeling interposers (e.g.flexible circuit cards) in between betavoltaic cells or groups of cells,or by many other methods common in the art. Various stacking andinterconnection configurations can be used to produce varying voltageand current outputs from the betavoltaic battery. See for example, thevarious approaches for connecting betavoltaic cells in series andparallel configurations in FIGS. 6-9 and the commonly-owned applicationSer. No. 15/602,078 referred to above.

Arranging multiple (N) layers of n/p cells in series with unidirectionalbeta sources is illustrated in FIG. 6. If it is assumed that all cellshave identical properties, namely, the same values for short circuitcurrent (I_(sc)), open circuit voltage (V_(oc)) and maximum power(P_(max)), and assuming the contacts between devices are ideal, thecharacteristics for the series stack of N cells are:

(I _(sc))_(stack) =I _(sc), (V _(oc))_(stack) =N×V _(oc), and (P_(max))_(stack) =N×P _(max)

Electrical connection between cells can be established by a soft metalsuch as indium or by a deposited peripheral strip of gold or anotherappropriate metal. Electrical contact can be made by contact pressurebetween metals, solders, electrically conductive epoxies, and othermethods well known in the art. FIG. 6 depicts an approach where theelectrical connections are made on the periphery of cells. Similarelectrical connections can be made for either n/p or p/n semiconductorlayers within the betavoltaic battery.

FIGS. 7A and 7B illustrate a novel approach for combining n/p and p/ncells in series with bidirectional beta sources, i.e., sources that emitbeta particles in two directions. This approach provides efficient useof a tritium layer in a bidirectional capacity. As shown in FIG. 7A, then and p layer dopants are reversed for the top and bottom cells,resulting in a negative polarity contact on the top surface and apositive polarity contact on the bottom surface. These contacts can beformed as discussed above for the series stack. If the cells haveidentical properties, except for polarity, the two-cell unit provides:

(Isc)unit=Isc, (Voc)unit=2×Voc and (Pmax)unit=2×Pmax

FIGS. 8A and 8B illustrate a configuration for combining two n/p (orp/n) cells in parallel and coupled to a bidirectional source. In thiscase, characteristics of the two-cell unit are:

(Isc)unit=2×Isc, (Voc)unit=Voc and (Pmax)unit=2×Pmax

Cells arranged in a stack but electrically connected in parallel isdepicted in FIG. 9 where such a structure of n/p (or p/n) cells, whereeach cell is coupled to a unidirectional beta source. Assuming there areN identical cells, each having Isc, Voc, and Pmax as cell parameters,

(I _(sc))_(stack) =N×I _(sc), (V _(oc))_(stack) =V _(oc), and (P_(max))_(stack) =N×P _(max)

The commonly-owned application Ser. No. 15/602,078 referred to above,describes several embodiments of series, parallel, and series/parallelconnected configurations.

Joining techniques (both electrical and physical) for stacks ofelectronic components (e.g., multi-chip stacking) such as, solderconnections, wire bonding, and other conductive adhesive materials andtechniques, can be utilized to join combinations of the configurationslisted in FIGS. 6-9. This allows for a broad variety of designinterconnection, thus achieving betavoltaic batteries with a variety ofcurrent and voltage specifications.

One embodiment of the present invention includes a method ofhermetically sealing a direct conversion semiconductor with tritiummetal hydride sources at low temperatures. During construction of thebattery and sealing of the package there is no leakage of tritium fromthe metal hydride, as would occur with high temperature sealing methodsdescribed elsewhere herein. Thus, this technique poses no risk oftritium exposure to the operator performing the sealing operation.Additionally, the hermetic battery design and the sealing techniquesallow for high throughput manufacturing and low contamination of tritiumwithin the manufacturing facility.

Hermetic packaging and sealing techniques for integrated circuits arewidely used in the semiconductor industry to prevent dirt, moisture,particulates and ionic impurities from entering the integrated circuitpackage and causing corrosion of the circuit elements and interconnects.In an embodiment of the present invention a combination of thesetechniques and packaging designs prevents tritium from escaping thebattery package. That is, the role of hermetic packaging and sealing forintegrated circuits is reversed in the case of the tritium battery, thatis, from contamination entering the IC package to preventing radioactivecontamination from exiting the tritium battery package.

In one embodiment of the present invention, the battery packagecomprises a ceramic or metal housing containing electrode pins or leadsextending from an internal area of the package to an external area ofthe package. See FIGS. 11A (external view of package), 11B (packageinterior with lid removed), and 11C (cross-sectional view of the packageinterior). The leads serve as conduits for electrical current generatedby the battery. The leads are hermetically attached and sealed via glassfrits or commonly used techniques for hermetic sealing of leads.Although the lead sealing methods involve high temperature processesabove 300° C., the leads are sealed on the battery housing prior tocontainment of the tritium metal hydride.

The semiconductor device (i.e., the n/p or p/n layers described herein)and tritium metal hydride source (comprising scandium, titanium,magnesium or other suitable metal tritide candidate) is inserted intothe package and connected to the leads via wire bonding or otherconventional techniques. The wire bonds are not illustrated in FIGS.11A-11C.

In one embodiment, the present invention uses a Kovar lid or a Kovarstep lid that closes the tritium battery package. If a ceramic packageis used a side brazed Kovar seal ring should be attached usingtechniques commonly known in the art. Note, the Kovar seal ring isattached prior to inserting the tritiated metal hydride.

The final step in completion of the betavoltaic cell is sealing of theKovar step lid to the metal package or to the ceramic side-brazedpackage, with a Kovar seal ring. See FIGS. 10A (bottom view) and 10B(side view). FIG. 10A illustrates a lid step and a lid edge. The lid issealed with a resistance or laser welder that uses localized heating,while the package remains well below 200° C., to hermetically weld thelid to the package. The preferred method for welding is a parallel seamwelder, which is inexpensive compared to laser welding and offers a highthroughput. Note, the most common method in the IC industry for hermeticsealing is the solder weld using a belt furnace. This method involvestemperatures of approximately 360° C., well above the threshold fortritium containment.

The tritium battery package seal is tested by enclosing the seam-sealerand the unsealed tritium battery package within a helium glove boxenvironment. Helium is flowed across the unsealed package and the Kovarlid is then placed on the package, trapping helium inside the package.The sealing is performed in glove box so that the trapped helium willremain within the package. The tritium battery package is then placed inan ultra-sensitive helium detector with detection levels up to 10̂-11cc/second under a 1 atmosphere differential. A helium leak rate of 10̂-8cc/second or less, under a 1 atmosphere differential is considered ahermetic seal for the tritium battery package. Such a leak rate iseasily achieved using this method. Additionally, lower hermiticityrequirements are still acceptable as long as tritium leakage is withinacceptable regulatory limits.

The package described herein may take any form of current IC packages,e.g., PIN device leads, leadless package, surface mounts, etc.

See commonly-owed U.S. Pat. Nos. 8,634,201 and 9,466,401 (both of whichare incorporated herein in the entirety) for further details of thesealing process.

In one embodiment, the battery package is constructed from machinedmetal parts e.g. aluminum, steel, titanium, and is welded or brazedusing techniques known to those skilled in the art to provide thehermetic seal. Electrical feedthrough(s) can be established usingstandard techniques in the electronics packaging industry, as describedabove.

There are also benefits to the operation and longevity of thebetavoltaic device that are directly derived from sealing the device inan inert atmosphere. Namely, the prevention of oxidation or corrosionreactions involving both the weld joint between the lid and package, aswell as oxidation that forms on the surfaces of the actual components ofthe betavoltaic device can be mitigated. Elimination of trapped oxygenand humidity though the use of a ultra-high-purity, very low humidity,inert gas prevents the possibility of generating an oxide product in theweld seal, which would produce an opportunity for tritium leakage out ofthe package, or humidity and oxygen leakage into the package.

Another approach to testing of the hermetic seal may be achieved with ahelium bombing system where the tritium battery package is enclosed inhigh-pressure helium environment. Depending on the size of the leakswithin the tritium battery package the helium gas will enter thepackage. The package is then removed from the high-pressure environmentand inserted in the ultra-sensitive helium detector unit to detecthelium leakage rates.

After a single betavoltaic cell (comprising the direct conversionsemiconductor layers and the tritium metal hydride source) is formed, aplurality of such cells may be connected in series, parallel orseries/parallel to achieve the desired current and/or voltage output.

In another embodiment containment of tritium and radiation emanatingfrom the tritium metal hydride is contained within individualizedtritiated direct-conversion semiconductor dies or epilayer dies. Thesedirect-conversion dies and tritium metal hydrides can be supplied withappropriate encapsulation that serves to contain the radiation.Encapsulation in the form of discrete, conformal coatings can be appliedthrough numerous techniques, such as dipping/immersion process,chemical/physical vapor deposition techniques, (e.g. potting,sputtering, evaporation, etc.). These coatings may be applied as thinfilms and can be polymeric, metallic or vitreous in nature orcombination thereof, providing some modest structural support androbustness to the direct conversion dies, while still providing animportant, necessary, and effective barrier to the emission of betaparticles arising from tritium decay and containment of the tritiumradioisotope.

Encapsulation is conducted to safeguard against any radiation leakage,but would be accomplished in a conformal manner so as to leave contactleads exposed as necessary for integration into device housings andmaintain geometric requirements for the dies.

These dies thusly encapsulated are then facile candidates for regulatorygeneral and/or exempt licensure; in this manner, the encapsulatedmaterials could easily be transported or handled without any risk ofradiation exposure and without any need for specialized radiationmaterials training. For example, the encapsulated tritium betavoltaicdies could be shipped to an OEM integrator for inclusion in anintegrated circuit package without a hermetic seal. Encapsulated diesmay be stacked or connected in series/parallel (using techniquesdescribed elsewhere herein or in the commonly-owed application Ser. No.15/602,078 (Attorney Docket Number 11432-010) referred to above) priorto or after encapsulation.

One aspect of the present invention involves increasing the surface areaper unit volume in a direct conversion device, without increasing thedark current, by using a surface texturing technique. See thecommonly-owned application filed on Jun. 24, 2014, assigned applicationSer. No. 14/313,953, and entitled Tritium Direct ConversionSemiconductor Device Having Increased Surface Area (Attorney Docket:11432-007), which is incorporated herein.

The ELO (epi layer liftoff) process has been referred to above. In oneembodiment, the ELO process is employed to remove an intact epilayercontaining the betavoltaic semiconductor device n/p or p/n layers. Theepilayer is approximately 0.1 microns to 5.0 microns thick, but can beas thick as 50 or 100 microns and is flexible. The epilayer isfabricated substantially free of surface defects that may be deleteriousto the betavoltaic device and thus increase the device dark current.

Such ELO devices may provide 0.1 to 0.2 microwatts of power per cm², butthe device may produce more or less power than these values, dependenton the device's active area and beta source strength. By stacking ofthese individual layers (as described elsewhere herein, the powerdensity can reach as high as 100-2000 microwatts/cm̂3, thereby achievingan increase in active surface area per unit volume resulting in asignificant increase in power per unit volume.

Epitaxial growth of the betavoltaic junction is accomplished on asubstrate using MOCVD or MBE processes. In the ELO approach the layersare grown in an inverted fashion. Thus the backing layer (also referredto as the back-side metallization) is on top surface. See FIG. 13A. Amaterial of the backing layer may comprise a metallic layer (e.g. gold,copper, aluminum, titanium, scandium, platinum, silver, tungsten, andother alloys) or a polymer material (e.g. polyimide, Kapton, etc.).

The backing layer provides structure and rigidity to the betavoltaicjunction layers as they are “lifted-off” from the substrate by etchingof a release layer. See FIG. 13B. With this process, the substrate maybe reused to grow another epilayer, thereby reducing the cost of thesubstrate material.

As shown in FIG. 13C, a temporary chuck/substrate is attached theback-side metallization layer to provide additional rigidity duringprocessing. The temporary chuck is removed by methods known in the art(e.g. dissolution of adhesion layer, selective etching, etc.).

The lifted-off betavoltaic junction cell may be further processed,tested, diced, or otherwise manipulated prior to release from thetemporary chuck. See FIG. 13D depicting a plurality of betavoltaic die.

Another fabrication technique, referred to herein as a substrate removalprocess is illustrated in FIGS. 14A-14D. According to this technique thejunction layers are grown in an upright orientation with an etch stopadded before BSFR layer possibly or before the nucleation/buffer layers.FIGS. 4 and 5 illustrate the placement of those layers.

After formation of the layers, the front or top surface of the structureis processed to create gridlines/contacts, isolation etch layer, metaldeposition, etc.) prior to tritium loading. See FIG. 14A.

The substrate is removed by etching away the entire thickness of thesubstrate (e.g., 500 microns), stopping at the etch stop layer, leavingthe epilayer betavoltaic junction, the contact metallization and thehydride metallization layers. See FIG. 14B.

Typically, a temporary chuck (see FIG. 14C) is affixed to the topsurface of the wafer (i.e. surface that may contain hydridemetallization or top/front contact layer) to provide rigidity andthereby prevent shriveling of the epilayers during subsequentprocessing. The etch stop layer shown in FIG. 14C may be retained orremoved through selective etching. The back-side metallization layer isformed and the temporary chuck removed.

Instead of removing the substrate using an etching process as describedabove, conventional substrate thinning techniques, such as mechanicalgrinding and polishing lapidary techniques can also be used. Theselapidary techniques can thin a substrate down to about 30 microns.

Another option substrate removal process uses a combinationmechanical/polishing of the wafer down to 30-50 microns combined withetching. The wafer requires support with a temporary chuck throughoutthe removal process.

It should be noted that the substrate referred to in the epilayerprocess may be undoped/insulating or doped. Typically, the substrate isdoped to make a back contact to the betavoltaic cell, but in the case ofa betavoltaic epilayer structure the contact may be directly made to thebetavoltaic epilayer after it has been released from the substrate. Thecontact can be made either through a metal contact or a metal tritideacting as a contact that is connected to a doped back surface fieldreflector layer or a doped buffer layer. The use of undoped/insulatingsubstrates offers a further cost reduction over doped counterpartsubstrates.

The back-side metallization layer referred to in the ELO and substrateremoval processes described above, may also serve as a source of betaparticles, i.e., a metal tritide. With two beta sources the creation ofEHPs within the epilayer can be approximately doubled. The effects ofthis doubling may be aided by the nascent thinness of the device and thelong diffusions lengths of the charge carriers in InGaP and other groupIII-V structures, thereby allowing for betavoltaic operation for betasentering through the base layer. Conductors in such an embodiment may beformed as a grid, allowing the beta particles to pass through openregions of the grid. In the event the metal tritide is not sufficientlythick to serve as a backing layer for the device, the metal tritide canbe fabricated with a greater thickness, by plating for example.

Moreover, a metal tritide formed on both sides of a betavoltaic epilayerprovides a symmetric distribution of forces under thermal expansion,thereby providing improved structural integrity for the betavoltaicepilayer. As described elsewhere, the metal tritide can also be sealedwith barrier layers (e.g. metallic, polymer, semiconductor, ceramicetc.) preventing the diffusion or migration of tritium or tritiumspecies out of the metal tritide and providing shielding againstradiation contamination.

In another embodiment, the combined epilayer and tritium metal hydride(comprising a thin betavoltaic device) may be stacked in series orparallel configurations as described in the commonly-owed applicationreferred to above, that is, application Ser. No. 15/602,078 (AttorneyDocket Number 11432-010).

In yet another embodiment, the tritium metal hydride may be formed on aseparate thin substrate or thin foil (e.g. less than 100 microns thick)and physically attached to the epilayer to form the betavoltaic device.

The epilayer described herein may comprise a III-V semiconductor and thebetavoltaic structure may have any of the constructions or combinationsthereof as described herein. For example, the betavoltaic epilayer mayhave a p/n or n/p structure with a doped or highly doped base, and a caplayer to protect the device from oxidation.

In one of the epilayer embodiments, it is also possible to establishcontact to a structure on a rear surface of the ELO betavoltaic devicethrough a top (i.e., front) surface of the device. This is accomplishedby chemically etching, physically ablating, or otherwise removingsemiconductor material from the top surface down to the metal backinglayer contact. See FIG. 12. This creates a contact to the metal backinglayer that is accessible from the top surface of the betavoltaic ELOdevice.

In another embodiment, an epilayer betavoltaic front contact isestablished to the cap layer and the rear contact (to present on thefront surface of the finished device) is in contact with an oppositepolarity layer (e.g., buffer, back surface field reflector, base, etc.).A portion of the top layers are chemically etched, physically ablated,or otherwise removed to expose one of the opposite-polarity layers. Theexposed portion may accommodate contacts in the form of metalcontacts/gridlines/ring/terminals/pads/points/epoxy(s)/solders etc.

In one embodiment, the released layers are coated on opposing faces witha radioisotope material (e.g. tritium hydride metal etc.) to allow betaflux to enter through opposing faces of the cell. This is particularlyuseful in cases where there is a monolithic SBU with two junctions.

It should be understood that any III-V direct conversion device may beformed into an epilayer and then released from its backing/substrate asdescribed.

In one embodiment a bilateral betavoltaic cell may be grown on asemiconductor substrate for use in the ELO process. For instance, foreither GaAs or Ge substrates a combination of a p/n and n/p betavoltaicstructure (e.g. InGaP, InAlP, InAlGaP, AlGaAs, or others) may be formedyielding twice the voltage of a single-sided counterpart. The bilateralcell is released from the substrate in a similar manner to the typicalELO process. Just as in the case of a single-sided betavoltaic, eachside of the bilateral ELO cell will require a source of beta fluximpinging on the respective junctions. The use of tunnel junctions maybe required to allow the current to flow in a series arrangement betweenthe two betavoltaic cells.

The various embodiments of the present invention allow construction of asingle flexible epilayer tritium betavoltaic battery or a very thinbetavoltaic battery that comprises a plurality of tritium betavoltaicepilayer cells stacked in either a series or parallel electricalconfiguration. For example, a thin epilayer tritium betavoltaic batterymay be constructed with either the tritium metal hydride film connectedto the epilayer or directly deposited on the epilayer. A thinbetavoltaic epilayer battery may be connected to a lithium ion thin filmbattery available from companies such as Front Edge Technologies ofBaldwin Park, California, Cymbet Corporation of Elk River, Minnesota,and Infinite Power Solutions from Littleton, Colorado. These twobatteries may be connected together as a joint film that may be pastedwithin an integrated circuit package to run the device periodically viapower bursts from the lithium thin film battery. The tritium epilayerbetavoltaic battery can trickle charge the lithium ion film battery.Periodically the lithium film battery can discharge power bursts atmilliwatt power levels and then be recharged via the trickle charging bythe tritium epilayer betavoltaic battery.

The tritium epilayer battery, due to its thinness and flexibility, maybe inserted into the conformal coating of an integrated circuit andstealthily power the integrated circuit. It can also be combined with alithium ion thin film battery into the conformal coating of anintegrated circuit as a source of power for the integrated circuit. Thetritium epilayer battery can also be placed within an integratedcircuit's package, multi-chip-module or printed circuit board or aceramic and/or metal hermetic package.

Some of today's secure processors and field programmable gate arrays(FPGA's) are using SRAM memory to store encryption keys. However currentbattery technologies depend on chemistries that are unreliable over longperiods of time (i.e. several years) especially under wide temperatureranges, such as −55° C. to +125° C.

The tritium betavoltaic batteries of the present invention are able topower the SRAM memory for periods of 15-20 years or more through theseextreme temperatures. Note, the voltage of tritium betavoltaic batteriesbased on III-V compounds will fluctuate less in higher temperatures thansilicon-junction based betavoltaic devices.

The tritium based betavoltaic batteries of this invention allowsoldier-to-base wireless communications and computer-to-basecommunication to be encrypted using FPGA's with encryption keys storedin SRAM as well as defense and telecom applications that experience awide range of temperatures. Note, the tritium betavoltaic batteries arehermetically sealed batteries packaged in surface mount packages thatmay be soldered to circuit board with the FPGA's

Another application of tritium based betavoltaic batteries of thepresent invention is for supplying power to anti-tamper volumeprotection for electronics and other devices that require protectionfrom intruders. These batteries provide the critical longevity of morethan 10 years for anti-tamper protection. Note, the temperatureresilience of these batteries is critical to the longevity andreliability. In one embodiment, a volume protection membrane from W. L.Gore is used on a circuit card to protect encryption keys stored in SRAMfrom a reverse engineering attack. The tritium betavoltaic batteries ofthis present invention may be hermetically sealed in a surface mountpackage and soldered on the circuit board to provide power to both thevolume protection device, the anti-tamper trigger in the processor andthe encryption keys held in SRAM. If an attack occurs on the volumeprotection device (i.e., W. L. Gore volume protection membrane), thetritium betavoltaic battery power allows the volume protection device todetect the attack and the anti-tamper trigger will erase all criticalinformation residing in the electronics, including the encryption keys.

Various layers are described herein as having a p-type dopant or ann-type dopant. Those skilled in the art recognize that the dopant typescan be reversed (n-type doped layers replaced with p-type and p-typedoped layers replaced with n-type) and the device will provide the samefunctionality.

Dopant concentrations are given for certain embodiments. Thesuperscripts + and ++ designate dopant concentrations that are greaterthan (+) and much greater than (++) conventional dopant concentrations.However, those skilled in the art recognize that different dopantconcentrations may be utilized to produce a functionalradioisotope-based direct energy conversion battery. In one sense thedopant concentrations are relative and dependent on the semiconductormaterial.

Also, certain embodiments have been described as having an intrinsiclayer; depending on the dopant types, doping levels, and other factors,this intrinsic layer may not be required in all embodiments. In theTables presented, the intrinsic layer is sometimes referred to as the“i-layer.”

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The scope of the invention mayinclude other examples that occur to those skilled in the art. Suchother examples are intended to be within the scope of the claims if theyhave structural elements that do not differ from the literal language ofthe claims, or if they include equivalent structural elements withinsubstantial differences from the literal languages of the claims.

In addition to the described embodiments and the layers comprising thoseembodiments, it should be noted that other embodiments of the inventionmay comprise one or more material layers from one of the describedembodiments used with one or more material layers from other ones of thedescribed embodiments.

In one embodiment, an alpha-radiation-emitting source can be utilized inlieu of a beta-emitting source, resulting in an alpha-voltaic that maymake use of one or more of the configurations described herein.

Another embodiment may comprise any combination of radioisotope sourcesin a single package, giving rise to combinations of alpha-, beta-, andgamma-voltaics. It should be noted that some choices of radioisotopesmay provide decay products that include radioisotopes that emit the sameor different types of radiation energy.

In one embodiment, the source may also be one which emits electrons as asecondary source (e.g. a particle strikes the source and the resultantemission comprises electrons). In addition, other radioisotopes orcombinations of radioisotopes and/or substrates whose end-product is anelectron or beta particle that impinges on the semiconductor may beutilized. The source may also be of a type that is intended to beattenuated to appropriate energy levels through the use of abarrier/filter.

In any of the embodiments above, the beta source can be substituted withan assembly that utilizes a radiation source impinging on ascintillating material for the purpose of generating photons that arecaptured by the cells. These radioisotope-indirect conversion devicescan take the place of the beta source in the device and generate photonemissions as well as transmit radioactive alpha, beta, or gammaemissions that may be captured by the junctions.

What is claimed is:
 1. A device for producing electricity, comprising: asupport material; a first plurality of stacked semiconductor materiallayers each doped a first dopant type and overlying the supportmaterial, one of the first plurality of material layers comprising aback surface field reflector layer for reflecting minority carriersreaching the back surface field reflector layer; a second plurality ofstacked semiconductor material layers each doped a second dopant typeand overlying the first plurality of semiconductor material layers, thesecond plurality of material layers comprising a window layer forreflecting minority carriers reaching the window layer a first contactin electrical contact with one of the first plurality of semiconductormaterial layers or in electrical contact with the support material; aradioisotope source proximate or in contact with an uppermost layer ofthe second plurality of semiconductor material layers, the radioisotopesource generating radioisotope particles or gamma rays; a second contactin electrical contact with one of the second plurality of semiconductormaterial layers; electricity produced between the first and secondcontacts by action of the radioisotope particles or the gamma rayswithin the device; and at least one layer of the first plurality ofmaterial layers having a bandgap greater than 1.8 eV and at least onelayer of the second plurality of material layers having a bandgapgreater than 1.8 eV.
 2. The device of claim 1 wherein an uppermost layerof the second plurality of stacked semiconductor material layerscomprises a cap layer and the support material comprises a conductivematerial or a doped material, wherein the first contact is in electricalcontact with the conductive material or the doped material, and thesecond contact is in electrical contact with the cap layer.
 3. Thedevice of claim 2 wherein the first contact and the second contact arepresent on a same surface of the device.
 4. The device of claim 1wherein one layer of the first plurality of stacked semiconductormaterial layers comprises a buffer layer and one layer of the secondplurality of stacked semiconductor material layers comprises a caplayer, wherein the first contact is in electrical contact with thebuffer layer and the second contact is in electrical contact with thecap layer, the first and second contacts present on a same surface ofthe device.
 5. The device of claim 1 wherein the radioisotope source isin contact with an uppermost layer of the second plurality ofsemiconductor material layers and comprises a metal tritide material,the second contact comprising the metal tritide material.
 6. The deviceof claim 1 wherein any two contacting layers of the first plurality ofstacked semiconductor material layers are lattice matched and any twocontacting layers of the second plurality of stacked semiconductormaterial layers are lattice matched.
 7. The device of claim 1 whereinthe support material comprises a Ge substrate, a lowermost layer of thefirst plurality of stacked semiconductor material layers in contact withthe Ge substrate for lattice matching the Ge substrate.
 8. The device ofclaim 1 where the radioisotope source comprises a material producingparticles or gamma rays for creating electron-hole pairs in one or moreof the first and second plurality of material layers.
 9. The device ofclaim 1 wherein the radioisotope source comprises a beta source furthercomprising tritium, nickel-63, promethium-147, tritium metal hydride, ora polymer containing tritium.
 10. The device of claim 1 wherein theradioisotope source comprises one of an alpha particle source, a betapolymer particle source, a gamma ray source.
 11. The device of claim 1wherein a material of one or more of the first and second plurality ofstacked semiconductor material layers comprises GaAs, InAlP, InGaP,InAlGaP, or AlGaAs.
 12. The device of claim 1 wherein one of the firstplurality of material layers comprises a base layer and one of thesecond plurality of material layers comprises an emitter layer, andwherein a material of the base layer comprises one of GaAs, InAlP,InGaP, InAlGaP, AlGaAs, and a material of the emitter layer comprisesone of GaAs, InAlP, InGaP, InAlGaP, AlGaAs.
 13. The device of claim 12further comprising an intrinsic layer between the base layer and theemitter layer.
 14. The device of claim 1 wherein the radioisotope sourcecomprises a metal tritide material.
 15. The device of claim 1 thesupport material comprising a metal backing, a tritide source, apolymer, an undoped substrate, or a doped substrate, a material of thedoped substrate further comprising germanium or GaAs.
 16. The device ofclaim 1 wherein a distance between the radioisotope source and aperimeter of the device is selected to be greater than an average traveldistance of a beta particle emitted from the radioisotope source. 17.The device of claim 1 wherein a material of a first layer in the firstplurality of stacked material layers is different from a material of asecond layer in the first plurality of stacked material layers, thematerial comprising one of GaAs, InAlP, InGaP, InAlGaP, and AlGaAs, andwherein a material of a first layer in the second plurality of stackedmaterial layers is different from a material of a second layer in thesecond plurality of stacked material layers, the material comprising oneof GaAs, InAlP, InGaP, InAlGaP, and AlGaAs.
 18. The device of claim 1wherein an uppermost layer of the second plurality of stackedsemiconductor material layers comprises a cap layer having a thicknessof less than 500 Angstroms, and wherein the cap layer is disposedoutside a region of the radioisotope source so as to not block theradioisotope particles or the gamma rays from reaching the secondplurality of stacked material layers.
 19. The device of claim 1 whereina material of one or more layers of the first and second plurality ofstacked semiconductor material layers comprises InGaP alloyed with 1-3%aluminum.
 20. A device for producing electricity, comprising: a supportmaterial; a first plurality of stacked semiconductor material layerseach doped a first dopant type and overlying the support material, oneof the first plurality of material layers comprising a first backsurface field reflector layer for reflecting carriers reaching the backsurface field reflector layer; a second plurality of stackedsemiconductor material layers each doped a second dopant type andoverlying the first plurality of semiconductor material layers, thesecond plurality of material layers comprising a first window layer forreflecting carriers reaching the window layer; a third plurality ofstacked semiconductor material layers each doped the second dopant type,one of the third plurality of material layers comprising a second windowlayer for reflecting carriers reaching the window layer; a fourthplurality of stacked semiconductor material layers each doped the firstdopant type and overlying the third plurality of semiconductor materiallayers, the fourth plurality of material layers comprising a second backsurface filed reflector layer for reflecting carriers reaching the backsurface filed reflector layer; a radioisotope source generatingradioisotope particles or gamma rays through opposing first and secondsurfaces thereof, the first surface of the radioisotope source proximateor in contact with an uppermost layer of the second plurality ofsemiconductor material layers, and the second surface of theradioisotope source proximate or in contact with a lowermost layer ofthe third plurality of semiconductor material layers; and at least onelayer of the first, second, third, and fourth plurality of materiallayers having a bandgap greater than 1.8 eV.